MIM stands for Metal Injection Molding. It is a powder metallurgical process that gained popularity after WW2. To somewhat over-simplify the process, it consists of taking a mold and placing steel powder into it. The steel powder is heated to a plastic, almost liquid state under high pressure. Once it has cooled enough to be released from the mold it can be heat-treated like any other steel part.
MIM has four major advantages and only one real disadvantage.
ADVANTAGES
- Lower cost
- Excellent dimensional tolerances
- Little additional machining necessary
- Far less hand fitting required
DISADVANTAGE
- Slightly lower tensile and shear strength than forged/machined steel
It would not be accurate to characterize MIM parts as "junk" as this is simply not the case. Like any other process if it is done incorrectly then an inferior product will result. If manufactured correctly then MIM parts are likely to last the lifetime of the gun. While it is true that most people would prefer to have forged, machined, and hand-fitted hammers and triggers in their guns, the extra labor costs involved make this prohibitive in today's competitive global environment. All the major gun manufacturers use MIM parts these days, including Colt, S&W, Kimber, Ruger, etc., so to pick on S&W exclusively is a bit ignorant.
People like to point out that MIM parts are not as strong as their forged/machined counterparts, but the strength differential is less than 10% and still well above the strength requirements of their intended function. Stamped steel is far less durable than MIM and yet wartime German P.38's made extensive use of stampings. Do we consider these as inferior weapons?
If it's really an issue then forged and machined parts are still available. Considering the high purchase price and the gunsmith's cost to install them it wouldn't surprise me to find that most of those who carp about MIM parts will choose to keep them in their gun rather than pony up the extra cash to get rid of the "junk".
-Lee
